Accurate high resolution, low temperature x-ray diffraction data will be collected on a selected series of iron porphyrins which model the active site of oxygen transport proteins. The data will be used to map in detail the electron density distributions of each of metal porphyrin complexes in each significant metal oxidation state spin state, and corrdination number. Several specific questions regarding the role of the iron heme in the cooperative mechanism for oxygen binding will be answered by analysis of the experimental charge distribution. The size of the iron atom in both the high-spin and low-spin states will be determined. The occupancies of the iron atom d-orbitals will be obtained from the aspherical distribution of density about the metal atom, providing information on the spin state and nature of bonding to the porphyrin ring and coordinated ligands. The experimental electron distribution of the porphyrin molecule provides, in addition, information of value to the understanding of this prosthetic group in other biological systems.